1. Filed of Invention
This invention relates to a method of evaluating segmental myocardial function from a value of contraction which is calculated from short-axis images of count distribution using radioactive myocardial perfusion imaging agents which accumulate in human heart after intravenous injection, and by using a rotating gamma camera under,electrocardiographic (ECG) gating.
2. Related Art
Assessment of myocardial function by using radioactive myocardial perfusion imaging agents has been frequently employed. This method includes estimation of myocardial wholesomeness by measuring myocardial blood perfusion and metabolism from the extent of accumulation of these agents. There are two widely employed techniques to evaluate the amount of accumulations: single photon emission computed tomography (SPECT) and positron emission computed tomography (PET). In both techniques, data are acquired from multiple angles around the object and transverse images are obtained. These techniques have advantages of acquiring more information than conventional planar images.
Another technique to assess the segmental cardiac function is to analyze ECG-gated images which are obtained by dividing one cardiac cycle into equally divided multiple time intervals. This ECG-gated technique enables us to obtain sequential SPECT or PET images from end-diastolic to end-systolic phases.
Changes in the wall thickness from diastole to systole are often employed to assess the myocardial function (maximum count method, K Narita, Kakulgaku 33 617-28, 1996, K Fukuchi, J Nuclear Medicine, 38 1067-73, 1997). Other methods to assess the cardiac function include: volume changes calculated using the edge-detection technique by thresholding, or by Gaussian approximation of the count distribution, or by approximating the contour of the inner surface of the chamber by a curve fitting (K Narita, Kakulgaku, 32, 1227-39, 1995, S Kumita, Kakulgaku, 33 1189-96, 1996, EG Depuey, J Nuclear Medicine 36, 952-5, 1995, Porenta, J Nuclear Medicine 36 1123-9, 1995), or by inspection of the epicardium by using three-dimensional animation of the wall motions.
In the maximum count method, the maximum count is determined by manually setting a region of interest (ROI), or by automatically picking up the maximum count along a line radiating radially from the center. However, there is debate that the point where the maximum count is measured at the systolic phase does not locate at the same position on the myocardium at the diastolic phase; also, the maximum count does not always indicate the thickness of the wall. The volumetric method has disadvantages of vagueness in determining the inner and the outer borders of the wall (I Buvat J Nuclear Medicine, 38 324-9 1997), and obtaining knowledge of the segmental wall function is therefore difficult.
The main purpose of the three-dimensional animation analysis is to study visually the wall motion which is centripetal, i.e., displacement of wall perpendicular to the wall. Poor wall motion which is observed by this method is not parallel to the wall, and therefore does not always indicate reduction of cardiac function at the point.
It is necessary to analyze the contraction parallel to the cardiac wall, because the muscle fibers are running nearly parallel to the cardiac wall. In this invention, muscle contractions parallel to the cardiac wall are evaluated from the displacement of each point on the wall by using images obtain by SPECT and PET.
From ECG-gated SPECT or PET images of radioactive myocardial perfusion imaging agents, contraction of the myocardium along the tangential direction, i.e., in the direction parallel to the myocardium, can be evaluated. The amount of the agents which accumulate in the heart muscle is calculated from reconstructed short-axis ECG-gated images and is projected on a virtual cylindrical screen whose axis is the same as the long axis of the heart. Spatial and temporal changes in time and space in the projected count distribution on the screen are described by a two-dimensional second order differential equation, which is called an equation of continuity for the fluid. This equation can be solved numerically with a computer. The displacements of points on the myocardium in the tangential direction are calculated by projecting the displacement of each point back onto the cardiac wall. From these displacements, contraction and expansion of the heart muscle can be quantified.